1. Field of the Invention
The present invention relates to an improvement of a fine feldspathic earthenware (cream ware) used as a tableware in restaurants, hotels, and homes, and an improvement of a process of manufacturing such a fine feldspathic earthenware.
2. Discussion of Related Art
Potteries and ceramics (whiteware) are generally classified by their water absorption percentage, since the characteristics of the potteries and ceramic are well represented by the water absorption percentage of their internal body. The Ceramics Division of the American Ceramics Society defines nine or ten kinds of potteries including: majolica having water absorption percentage of not lower than 15%; a fine feldspathic earthenware having water absorption percentage of not lower than 3% and lower than 15%; and a stoneware having water absorption percentage of not lower than 0.1% and lower than 3%. The former version of JIS S3001-3003 (Japanese Industrial Standard) used a similar classification of the potteries according to the water absorption percentage.
In Japan, the potteries the bodies of which have water absorption percentage of 3–15% are referred to as “fine feldspathic earthenware (cream ware)”, “feldspathic earthenware”, “fine earthenware” or “semi-vitrified ceramics (earthenware)”. In the United States, the United Kingdom and other European countries, those potteries are referred to as “earthenware” or “semi-vitreous china”, for example. These potteries are usually or commonly used in restaurants, hotels and homes. Such potteries (hereinafter referred to simply as “fine feldspathic earthenware”) have bodies whose surface is covered with a glaze layer, and have the following characteristics: (a) a high mechanical strength and high ease of handling, as well as a comparatively small weight owing to a specific gravity of about 2.0; (b) particularly high degrees of heat or thermal insulation and thermal shock resistance, owing to the formation of multiple pores in the body, and safe use for cooking with an electronic or gas oven; (c) a sufficiently high degree of whiteness even in the case of using a comparatively inexpensive material, owing to the firing at a temperature not so high as to achieve full vitrification in the process of manufacture; and (d) a high degree of durability without removal of a glaze color or pigment, owing to a hard glaze layer covering the surface of the body, in the absence of an overglaze decoration, with only an underglaze decoration given on the body surface.
Referring to the schematic views of FIGS. 1A–1E in cross section illustrating major process steps of manufacturing a fine feldspathic earthenware in the prior art, the process steps include a step of placing a body on a so-called rollerhead machine 10 equipped with a gypsum mold, and press-forming the body into a plate-like or dish-like body. FIG. 1A shows this step. The thus formed bodies 12 each in the form of a plate or dish are dried for a sufficient time, and are then biscuit-fired at a temperature of about 1100–1300° C. in a kiln such that the formed bodies 12 are superposed on each other in a stack on a setter 14. FIG. 1B shows this step. Successively, each biscuit-fired body 16 is given an underglaze decoration, which is transferred from a transfer paper bonded to its surface or drawn with a suitable powdered pigment. Then, the biscuit-fired body 16 is supported by pins 18 at a plurality of positions (e.g., three positions) on its back surface, and the surfaces of the body 16 are uniformly coated with a glaze, with spray guns or sprayers 20. FIG. 1 C shows this step. After the body 16 coated with the glaze is sufficiently dried, the body 16 is subjected to a glost firing operation, with a refractory jig 22 as described in JP-U-54-4263. For example, this jig 22 is a generally annular structure having a plurality of projections 24 (e.g., three prismatic pins each in the form of a triangular prism having a length of a several tens of millimeter and three faces each having a width of several millimeters). The projections 24 protrude inwardly of the annular structure of the jig 22. The biscuit-fired body 16 is supported by the jig 22, with the projections 24 held in abutting contact with the back surface of the body 16. In this condition, the body 16 is glost-fired at a temperature of about 900–1200° C. FIG. 1D shows this step. Subsequently, the glost-fired fine feldspathic earthenware 26 is separated from the projections 24 of the jig 22. Usually, fragments 28 of the projections 24 adhere to the earthenware 26 as a result of a reaction of the projections 24 with the glaze. These fragments 28 are removed with a grinding wheel 30 which carry a grinding layer formed of abrasives of diamond, silicon carbide or corundum. FIG. 1E shows this step. Thus, the fine feldspathic earthenware 26 is manufactured.
2A and 2B show in detail the step of removing the fragments 28 of the projections 24 which remain adhering to the fine feldspathic earthenware as a result of the reaction of the projections 24 with the glaze in the glost-firing step, after the glost-fired fine feldspathic earthenware 26 is removed from the projections 24 of the jig 22 described above. FIG. 2A shows the earthenware 26 before removal of the fragments 28, while FIG. 2B shows the earthenware 26 after the removal. The bottom plan view of FIG. 3 shows the fine feldspathic earthenware 26 after the removal of the fragments 28. FIG. 2B is a cross sectional view taken along line 2—2 of FIG. 3. As shown in FIGS. 2A, 2B and 3, the fine feldspathic earthenware 26 manufactured in the prior art process inevitably suffers from pin marks 32 caused by the pins 18 used to support the biscuit-fired body 16 in the glaze coating step, and ground spots 34 caused by the removal of the fragments 28 of the jig 22. These pin marks 32 and ground spots 34 have non-glazed surface areas which are not coated with the glaze and which have a relatively high water absorption percentage. In particular, the ground spots 34 have a comparatively large diameter of about 2–5 mm, and accordingly give rise to various problems.
Described in detail, the fine feldspathic earthenware whose body has a relatively high water absorption percentage is ideally covered over its entire surface with the glaze layer. Namely, the body of the fine feldspathic earthenware is likely to absorb contaminated liquids or particulate matters, at its exposed non-glazed areas, and these liquids and particulate matters tend to stay in the pores existing adjacent to the surfaces of the exposed non-glazed areas. Accordingly, the contaminated non-glazed areas are unfavorably contrasted with the other area of the body or the glazed area, which is transparent, or white- or otherwise-colored uniformly. Thus, the contaminated non-glazed areas highly possibly degrade the appearance of the earthenware. In addition, particles are produced when the fragments 28 of the projections 24 of the jig 22 are removed by the grinding wheel 30 after the fine feldspathic earthenware 26 is separated from the jig 22. Since the particles may adhere to the other pieces of earthenware 26 under manufacture together with the piece in question, the individual pieces of earthenware 26 under manufacture must be isolated from each other, to prevent the adhesion of the particles produced in the grinding operation. Further, the grinding operation which must be performed manually inevitably increases the cost of manufacture of the earthenware 26 and lowers the efficiency of the manufacture, and may damage the earthenware 26 to a commercially unacceptable extent, due to an error in the manual grinding operation. The fragments 28 remain on the fine feldspathic earthenware 26 immediately after the removal from the kiln after the glost-firing operation, may have a risk of injuring the fingers or other parts of the worker, when those parts contact sharp edges of the fragments 28.
The pin marks 32 are comparatively small, and are not highly perceptible while the fine feldspathic earthenware 26 is still new. As the earthenware 26 is aged, however, exposed non-glazed areas provided by the pin marks 32 are gradually contaminated and contrasted with the adjacent glazed, colored areas, and unfavorably degrade the appearance, like the ground spots 34. Further, the non-glazed areas of the pin marks 32 have a higher surface roughness, than the glazed area, and may mar the upper surface of the underlying piece of earthenware 26, when the pieces are stacked for transportation. In addition, the pin marks 32 may be a source of cracking or breakage of the earthenware 26.